The fullscreen scale modes (scale/ Letterbox scale/ Letterbox integer scale) refer to each layers scale rates, instead of being separate.It basically means if you have a game with variation in scale rates of layers, the fullscreen scale modes won't work properly.

Simple example: a HUD layer with scale rate set to zero won't scale for the games zoom (which is correct) but also means it doesn't scale to meet the window size with fullscreen modes on.